Abstract Zirconium boride (ZrB 2 ) whiskers hold significant potential for toughening ultra‐high temperature ceramics (UHTCs). Yet, their fabrication remains challenging due to limited knowledge of the microstructure evolution. Herein, high‐purity ZrB 2 whiskers were successfully prepared by electrospinning combined with carbothermal reduction. Specifically, a precursor solution containing Zr–O–C–B networks stabilized through hydrogen bonds with polyvinyl pyrrolidone was synthesized as a spinnable ZrB 2 precursor. High‐purity ZrB 2 whiskers with a diameter of ca . 200 nm and a length of 5–10 µm were obtained by precise heat treatment of ZrB 2 precursor fibers. In this process, ZrO 2 grains formed initially and transformed into ZrB 2 grains through carbothermal reduction, which further converted into ZrB 2 whiskers through a solid‐liquid‐solid mechanism. To testify to the toughening effect for UHTCs, the ZrB 2 whiskers were added into ZrB 2 powders and followed by sintering. This strategic addition of 5 wt% ZrB 2 whiskers led to a 19% enhancement in the fracture toughness of ZrB 2 bulk. This study illuminated the preparation strategy and formation mechanism for ZrB 2 whiskers and their application for toughening UHTCs.